In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transreflective type utilizing both the natural light and the backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship between two of the general characteristics. The general characteristics of the composition will be further explained based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferable maximum temperature of the nematic phase is about 70° C. or higher and a preferable minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferable for displaying moving images on the device. Accordingly, a small viscosity in the composition is preferable. A small viscosity at a low temperature is further preferable.
TABLE 1General Characteristics of Composition and AM DeviceGeneral Characteristics General CharacteristicsNo.of Compositionof AM Device1wide temperature range of a wide usable temperature rangenematic phase2small viscosity 1)short response time3suitable optical anisotropylarge contrast ratio4large positive or negative low threshold voltage and small electricdielectric anisotropypower consumption large contrast ratio5large specific resistancelarge voltage holding ratio and largecontrast ratio6high stability to ultraviolet long service lifelight and heat7large elastic constantshort response time large contrast ratio1) A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
An optical anisotropy of the composition relates to a contrast ratio in the device. A product (Δn×d) of an optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is in the range of about 0.30 micrometer to about 0.40 micrometer in a device having the VA mode, and in the range of about 0.20 micrometer to about 0.30 micrometer in a device having the IPS mode. In the above case, a composition having a large optical anisotropy is preferable for a device having a small cell gap. A large absolute value of a dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large absolute value of the dielectric anisotropy is preferable. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in the device. Accordingly, a composition having a large specific resistance, at room temperature and also at a high temperature in an initial stage, is preferable. A composition having a large specific resistance, at room temperature and also at a high temperature after the device has been used for a long time, is preferable. Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. Such characteristics are preferable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
A composition having a positive dielectric anisotropy is used for an AM device having the TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the PSA mode. Examples of the liquid crystal composition having the negative dielectric anisotropy are disclosed in Patent literatures No. 1 as described in the following. Examples of the liquid crystal composition for STN mode having the positive dielectric anisotropy are disclosed in Patent literatures No. 2 and 3.